It is known to control braking effort by providing pressurized brake fluid, which is applied to brake cylinders under control of electromagnetic valves. A controller is provided, which is responsive to various switching thresholds to control the electromagnetic valves. The electromagnetic valves are located between a pressure source and one or more of the vehicle brakes themselves, and control, respectively, increase in braking pressure, decrease of braking pressure, for example by drainage of brake fluid, and maintenance of a certain braking pressure, and hence certain braking effort, by closing a fluid circuit to the brake so that the brake fluid under the previously provided pressure is trapped therein and continues to apply the braking pressure, maintaining it constant.
Braking systems of the type generally described above are described in the literature, in which respective parameters, such as actual braking pressure P.sub.B, or vehicle deceleration V.sub.F or wheel slippage is used as a comparison parameter to indicate braking effort. U.S. Pat. No. 3,790,225 discloses a system in which actual braking pressure P.sub.B is the actual braking effort parameter; GB-PS No. 1465613 discloses a system in which vehicle deceleration is the actual effect parameter.
Brake control systems may use pressurized hydraulic fluid or compressed air as the pressure medium; some systems use a combination of both. Some time delay is unavoidable between sensing of a command value, that is, operation of a command element such as a brake pedal, and the beginning of build-up of brake fluid pressure. As the brake fluid pressure increases, the pressure sensor provides a feedback or response signal representative of actual pressure. When the command value determines the pressure to be commanded, a delay will arise before the actual pressure will reach the commanded value. This delay is due to the effect of hydraulic or pneumatic connection lines, delayed in operation of the electrically controlled valves which control flow of brake fluid, and the like. Consequently, brake control systems of the simple type in which only a comparison between actual braking pressure and commanded braking pressure is made, will result in hunting of the actual braking pressure level about the command value. Such hunting causes repeated operation of the magnetic valve for, respectively, admitting and draining pressurized brake fluid.